1. Field of the Invention
The present invention relates to an image forming apparatus that forms a toner image by selectively transferring the toner onto a latent image due to differences of electrostatic potential, transfers the toner image onto the recording material, and then forms a fixed image by heating and pressing the toner image.
2. Description of the Related Art
In an image forming apparatus using a powdered toner, a process of fixing a toner image generally includes electrostatically transferring the toner image directly onto a recording material, or primarily transferring the toner image onto an intermediate transfer body and secondarily transferring it onto the recording material, then heating and melting the toner containing a thermoplastic resin binder and transferring it onto the recording material. This thermal fixing method includes passing the recording material carrying an unfixed toner image a nip part between a heating rotatable member and a pressing rotatable member contacted and pressed to fix the toner image, and is most widely employed because of its advantages that the toner image can be fixed at low electric power, and there is less danger of burning due to paper jam in a fixing part.
The heating rotatable member has a hollow core bar of aluminum or the like, and a heater provided inside it, and is covered with fluororesin on the surface to improve the release characteristics. Also, the pressing rotatable member is provided with an elastic layer on a metallic core bar, and covered with a fluororesin tube having good release characteristics on the surface layer.
Since the fluororesin based material has high electrical insulating property and is easily charged, when the recording material with unfixed toner is passed through the nip part, there occurs a phenomenon that a part of the unfixed toner on the recording sheet adheres to the surface of the heating rotatable member, causing an offset. The offset toner adheres to the recording material to be conveyed next, contaminating the recording material, or causing an image defect.
Besides, it has been newly found that one of the causes that the toner is offset to the heating rotatable member is that the paper dust adhering to the heating rotatable member induces the offset. Accordingly, there is a demand for means for preventing the offset caused by the paper dust effectively.
Recently, the recycled paper is often used from the viewpoint of the forest resource maintenance or the global environment maintenance. Also, the inferior paper (called “low cost paper”) have been often used to reduce the cost of the recording material. Since more paper dust may separably adhere to the recycled paper or the low cost paper, a part of paper dust may often adhere to the heating rotatable member, when employed in the image forming apparatus. And when the recording material carrying the unfixed toner is passed through a nip part between the heating rotatable member and the pressing rotatable member, the unfixed toner is transferred to the paper dust adhering to the heating rotatable member, causing the offset of the toner around the peripheral surface of the heating rotatable member. Especially when fixing the image having low image density and a large white portion to which no toner is transferred, the paper dust existing in the white portion is likely to adhere to the heating rotatable member. The adhering paper dust induces the offset of the toner, often leading to an image contamination.